The complex formation of europium(III) with the zwitterionic form of amino acids (alanine, phenylalanine, and threonine) has been studied in aqueous solution. Measurements were performed at <i>I</i> = 0.1 M (NaCl/NaClO<sub>4</sub>), room temperature, and trace metal concentrations in the range of pH 2 to 8 using ultraviolet–visible (UV-Vis) and time-resolved laser-induced fluorescence spectroscopy (TRLFS). While complexation leads to a significant luminescence enhancement in the emission spectrum of the metal ion, absorption in the UV-Vis spectrum of the amino acid (AA) decreases. As zwitterionic species (AAH), all three ligands form weak complexes with 1:1 stoichiometry and a general formula of EuAAH<sup>3+</sup> with the metal. The complex stability constants were determined to be log <i>K</i> ∼ 1 for all complexes, indicating the negligible contribution of the amino acid side chain to the complex formation reaction.
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